1. Field of the Invention
The present invention relates to an Ethernet transmission apparatus which has a function of switching an operational system and non-operational system, and particularly to an Ethernet transmission apparatus for preventing the same frame from being transmitted multiple times.
2. Description of the Related Art
In the 1+1 Bidirectional mode of Ethernet Linear Protection recommended in ITU-T G.8031, the connection between transmission apparatuses is duplexed by Working/Protection transmission lines which serve as operational/non-operational systems, and either one of the Working/Protection transmission lines is selected to be used for transmission depending on the status of the transmission line.
The transmission side MEP (Maintenance group End Point) transmits ETH-CC (Continuity Check) at regular intervals to both of the Working/Protection transmission lines independently, and the receiving side administers only the status of the transmission lines by using ETH-CC and performs switching of the transmission lines according to an alarm state.
Transmission technologies which exploit the switching of operational and non-operational systems include those disclosed in Japanese Patent Laid-Open No. 2006-174406, Japanese Patent Laid-Open No. 2002-354011, and Japanese Patent Laid-Open No. 08-237253.
Japanese Patent Laid-Open No. 2006-174406 discloses a method and apparatus which aims at realizing highly-reliable failure recovery without packet loss. Specifically, there is disclosed in FIG. 28 and Paragraph 0125, which refers to FIG. 28, a configuration in which a counter is transmitted into Inter Frame Gap (IFG) to measure the counter delay difference between active and standby systems so that the slower one is discarded.
Japanese Patent Laid-Open No. 2002-354011 discloses a technique which aims at performing switching of the transmission line without causing loss and unsynchronized switching of undefined packet lengths in a duplexed switching system in which the undefined packet lengths are mapped to SDH (Synchronous Digital Hierarchy) frames, the technique being configured such that the same SDH frame in which an undefined length packet is inserted into a payload portion according to the timing based on the phase of the SDH frame is transmitted through the operational and non-operational systems.
Japanese Patent Laid-Open No. 08-237253 discloses a technique which also aims at realizing synchronized switching without instantaneous interruption when switching duplexed, active and standby virtual paths as with the technique of Japanese Patent Laid-Open No. 2002-354011, and in which an OAM (Operation Administration and Maintenance) cell to which a sequence number and an identification code for the necessity of switching have been added is intermittently transmitted over duplex virtual paths, and the sequence number is checked to detect differential delay and loss of OAM cell so that synchronization is established and the virtual paths are switched.
Among the techniques described in each of the above described patent documents, the technique disclosed in Japanese Patent Laid-Open No. 2006-174406 utilizes IFG and cannot be used in general Ethernet networks. Moreover, configuring the technique disclosed in Japanese Patent Laid-Open No. 2006-174406, in which an identifier is given to each frame, requires complicated processing and the transfer operation requires much time.
The techniques disclosed in Japanese Patent Laid-Open No. 2002-354011 and Japanese Patent Laid-Open No. 08-237253 aim at the realization of synchronized switching, and detection of packet loss is a common problem in each of the patent documents.
In Ethernet, in the case of a duplex system (1+1 Protection), even if a packet disappears during the switching of operational/non operational system, the disappeared packet that disappeared can be recovered in an upper layer and therefore will not cause a serious problem.
When there is a difference in propagation time between each of the Working/Protection transmission lines due to a path length difference, a Node number difference and the like, the same frame will not be received at the receiving side at the same time. When switching is performed from the Working side to the Protection side with the Protection side being delayed, the same frame will be invariably outputted from the receiving side. Occurrence of redundant packets will cause the transmission line to be crowded, resulting in congestion. Since such a redundant packet problem, in which two or more of the same packets are received, is not taken into account in Ethernet, it is an essential problem to be avoided.
3. Summary of the Invention
It is an object of the present invention to provide a technique for preventing a plurality of the same frames from being received during the switching of operational/non-operational systems at the time of a malfunction of an Ethernet configured for redundancy.
The Ethernet transmission apparatus of the present invention comprises a first and second MEP provided for each of the transmission lines of operational/non-operational systems and for performing the transmission/reception of a frame to be transmitted over each transmission line in a duplexed manner, and a switching processing part for switching the MEP to be used for reception by using ETH-CC which is transmitted at regular intervals, and the Ethernet transmission apparatus is characterized in that
the above described first and second MEPs are configured to be able to confirm each other's received content, and are adapted to add a continuous sequence number to the ETH-CC when transmitting the ETH-CC, and that, when the sequence number of the received ETH-CC is smaller than the sequence number of the ETH-CC received by the operational system side, the MEP of the non-operational system side is adapted not to transmit the above described frame.
The Ethernet transmission system of the present invention comprises the above described Ethernet transmission apparatus and transmission lines for operational and non-operational systems.
The Ethernet transmission method of the present invention is performed in an Ethernet transmission apparatus comprising a first and second MEPs provided for each of the transmission lines of operational and non-operational systems, for performing the transmission/reception of a frame to be transmitted over each transmission line in a duplexed manner, and configured to be able to confirm each other's received content, and a switching processing part for switching the MEP to be used for reception by using ETH-CC which is transmitted at regular intervals, and the transmission method is characterized by comprising:
configuring the above described first and second MEPs to add a continuous sequence number to the above described ETH-CC when transmitting the above described ETH-CC; and adapting the MEP of the non-operational system side not to transmit the above described frame when the sequence number of the received ETH-CC is smaller than the sequence number of the ETH-CC received by the operational system side.
In the present invention configured as described above, since the MEP which receives ETH-CC having a smaller sequence number does not transmit the frame, the frame to be sent to the switching processing part will be a newer frame having a larger sequence number. As the result of this, there will be no chance that frames of the same content will be sent to the switching processing part, and thereby a rapid transfer of frames is enabled.